Ag Crocker

Models to describe brittle and ductile fracture in ferritic steels

Abstract Theoretical models have been developed to describe the various fracture mechanisms that occur in polycrystalline α-iron and ferritic steels over a range of temperature which spans the ductile-to-brittle transition. At low temperatures, the models enable the proportions of transgranular cleavage and intergranular brittle failure to be explored for different ratios of the fracture energies for the two mechanisms. This allows, for example, the effect of embrittlement arising from grain-boundary segregation of minor alloying and impurity elements to be investigated. In addition the effect of texture on the predictions obtained has been considered and the results are presented. At higher temperatures, the models have been developed to accommodate ductile fracture. They have also been extended to consider the influence of prior creep cavitational damage at grain boundaries on both low- and high-temperature fracture processes and the corresponding fracture energies. The effect of this prior damage on subsequent ductile failure is explored. The predictions show that, if fewer than about 20% of the grain boundaries are fully cavitated, then there is little change in either the mechanism of the fracture process or the fracture strength of the material. These predictions are compared with measured upper-shelf fracture toughness values obtained for a Cr-Mo-V ferritic steel containing proportions of up to about 40% of prior creep damage.

An examination of the linkage of cleavage cracks at grain boundaries

Abstract Grain boundaries resist the propagation of cleavage cracks in polycrystalline materials, and 3D geometrical models have been used to predict the accommodation required at a grain boundary as a crack propagates from grain to grain. This paper describes how focused ion beam (FIB) microscopy, which provides topographic and crystallographic contrast imaging and allows ion milling to be undertaken at selected areas of interest, can be used to investigate these local fracture events. Results of low temperature fracture of polycrystalline bcc Fe–3%Si and hcp zinc are presented. The interactions between these results and the geometrical modelling are briefly discussed.

Understanding fracture behaviour of PGA reactor core graphite: perspective

Abstract Magnox reactors are cooled by carbon dioxide gas. The pile grade A (PGA) graphite moderator bricks in the reactor core loose mass and become more porous during service due to the radiolytic oxidation caused by energy deposition, mainly gamma radiation. In addition, neutron irradiation brings about strengthening by irradiation hardening and dimensional change. In this perspective, experimental data related to the attendant microstructural changes and the associated initiation and propagation of cracks within the graphite are revisited. These results are compared with the predictions of multiscale finite element modelling based upon an idealised microstructure. The discussion considers the quasi-brittle characteristics of the PGA graphite over a range of service exposure conditions.

Computer Modelling of Crack Propagation in Porous Reactor Core Graphite

This study aims to develop computer models, with a microstructure representative of the PGA graphite, to contribute to the understanding of the relationship between the amount of porosity, the load-displacement behaviour and crack propagation. The project is in two linked parts, the first provides a model of the porous graphite which is then introduced into a lattice type finite element model to provide the load-displacement and crack propagation predictions. Microstructures consisting of matrix and pores with added aligned filler particles, typical of needle coke, were studied. The purpose was to isolate the effect of filler particles on fracture strength and the fracture path. In the paper crack paths and fracture mechanisms are discussed for different amounts of porosity and various filler particle arrangements.

Three-Dimensional Modelling of Fracture in Polycrystals

A great deal of research has been carried out on modelling the fracture of polycrystalline materials. However, this has been largely restricted to 2-D models and special cases of 3-D models, e.g. Smith et al. [1], Crocker et al. [2]. Using 2-D models, brittle cleavage cracks in adjacent grains meet at a point in their common grain boundary. A crack can therefore propagate from grain to grain without the necessity of any grain boundary failure. In real materials this is not the case. Cleavage cracks in adjacent grains do not in general meet in a line in their common grain boundary so that some grain boundary failure or an equivalent accommodation mechanism, such as multiple cleavage or ductile tearing, must occur. For this and many other reasons it is important to develop satisfactory 3-D models, Smith et al. [3]. The simplest that have been used are in the form of a body-centred cubic array of identical, regular tetrakaidecahedra (14-hedra). These have suggested that of the order of 30% of brittle fracture is in the form of accommodation grain-boundary failure. This does not include complete grain boundaries that fail because of some inherent weakness. The figure of 30% is much greater than most reported experimental values and therefore there is a need for more realistic 3-D models. One approach has been to construct prismatic grains on a random array of 2-D polygonal cells. This is a good approximation of the structure of columnar grains in weld metal and again suggests that in brittle fracture about 30% of accommodating grain boundary failure is needed.[1] However, as the prisms are effectively of infinite height, this figure gradually increases as fracture propagates.

Effect of prior creep cavitation on brittle fracture in heat affected zone of ferritic steel weldments

Abstract Many components used by a range of industries, such as electrical power generation, operate at temperatures where creep damage is accumulated during the service life. In this process the materials undergo a range of microstructural changes including accumulation of cavities on grain boundaries oriented predominantly normal to the direction of the maximum principal stress. A theoretical model has been developed to consider the influence of such prior creep cavitational damage on subsequent brittle fracture of ferritic steels. The model allows the modes of transgranular cleavage and intergranular fracture to be quantified. It shows that if there is less than ~20% of cavitated boundaries they have little effect on the nature of the fracture process or the fracture strength. Material encompassing the heat affected zone has been removed from a region of a manual metal arc weldment in a CrMoV steel containing creep cavities arising from stress relief heat treatment. Fracture toughness tests using reconstituted Charpy geometry specimens were undertaken at a temperature of 170 K to assess the effect of this damage on the lower shelf fracture toughness. The results are discussed with respect to the predictions of the theoretical model.

The Brittle Fracture of Polycrystalline Zinc

In polycrystalline materials grain boundaries provide an important contribution to the resistance to the propagation of both brittle and ductile cracks. In this paper we describe experimental measurements of brittle cracks developed within both small punch and matchstick test specimens of polycrystalline hcp zinc. These specimens were tested over the temperature range 77 to 423K. Fractography undertaken using focussed ion beam imaging provides detail of the propagation from grain to grain and across {10-12} twins of (0001) basal and {10-10} prismatic cleavage cracks. The results are discussed by comparison with the predictions from previously described 3-D geometric modelling applied to this hcp polycrystalline material.

An experimental and modelling study of brittle cleavage crack propagation in transformable ferritic steel

Abstract Flaws that can be present within pressure vessels, pipework and other engineering structures are assessed using the principles of engineering fracture mechanics. It is necessary to support such an approach with an understanding of the underlying fracture mechanisms. Moreover, many of these components are fabricated using transformable steels. In the present paper, the authors describe the fracture of an A508 type steel, heat treated to produce a tempered bainitic microstructure, and subsequently impact tested at −196°C. In particular, focused ion beam microscopy has been used to produce high resolution fractography, combined with information relating to the underlying microstructure and crystallography. The results of cleavage crack propagation across prior austenite grain, lath packet and lath boundaries are described and then correlated with predictions from a three-dimensional geometric model of brittle cleavage fracture in polycrystalline steel. This model includes a consideration of a lath substructure developed within the grains and is based upon a Kurdjumov–Sachs orientation relationship with the parent austenite grain.

A Consideration of Cleavage Crack Propagation in Fe2Si Steel

It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.

An experimental and theoretical consideration of the effect of prior creep damage on the heat affected zone fracture toughness of CrMoV steel

Material encompassing the heat affected zone has been removed from a region of a manual metal arc weldment in a CrMoV steel. This zone contained creep cavities on grain boundaries oriented normal to the direction of the maximum principal stress arising from the stress relief heat treatment. Fracture toughness tests using reconstituted Charpy geometry specimens with side grooves were undertaken at temperatures of 150 and 350 °C to assess the effect of the creep cavitation on the measured fracture toughness. The results are compared and discussed with respect to the predictions of a theoretical two-dimensional model.